The invention concerns a method for the electrothermal measurement of level which is compensated for ambient temperature and in which a resistance probe is heated with a constant current in each case during a heating time, a first variable formed from at least one probe voltage at the start of the heating time is compared with a second variable formed from at least one probe voltage at the end of the heating time in order to form a comparison variable which is approximately proportional to the level, the forming of at least one of the two variables being effected in accordance with a time function.
Another aspect of the invention concerns a circuit for the electrothermal measurement of level compensated for ambient temperature, having a switched source of constant current on a resistance probe, a comparator which compares a first variable formed from at least one probe voltage at the start of the heating time with a second variable formed from at least one probe voltage at the end of the heating time, and means for forming at least one of the two variables in accordance with a time function.
It is known that the level of liquids can be measured with a resistance probe which consists of an elongated temperature-dependent resistor of positive temperature coefficient. This resistance probe is heated by a constant current. The heat produced in this way is given off to the environment corresponding to a heat transfer resistance which is dependent on the extent to which the resistance probe is immersed in the liquid medium or surrounded by the gaseous medium. The heating of the resistance probe results in an increase in its resistance during the heating time. The change of the resistance with time depends in this connection on the initial resistance of the resistance probe, the temperature coefficient, the constant current and the heat transfer resistance. The dependence on temperature of the initial resistance at the start of the constant current, when the latter can still not affect the increase in resistance, must be compensated for in order to obtain more accurate measurements of the level.
This will be discussed further below. Assuming that the temperature coefficient and the constant current are constant during the measurement, the variation with time of the resistance is dependent on the heat transfer resistance and the depth of immersion in the liquid medium. From the change in resistance there results a change in the probe voltage, which change is evaluated. The evaluation also affects the precision of the measurement. The methods of evaluation known up to the present time utilizethe change with time of the probe voltage by a differentiation or by determining the voltage difference after a given period of time, i.e. as a rule at the start and end of the heating time, in order to determine the depth of immersion.
If the resistance probe is allowed to cool down for a sufficiently long period of time by disconnecting the constant current, then repeated measurements during measurement periods are possible.
In one such known circuit, the resistance probe is connected, controlled by a program control (delay circuit), upon the commencement of each measurement (commencement of a measurement period) to a source of constant current which remains connected during the heating time. The delay circuit is connected to a comparator. The comparator is connected, on the one hand, to an inverter which is connected to the resistance probe and a resistor/capacitor member connected in parallel thereto, in such a manner that the comparator gives off an output signal to a monitoring device as soon as the voltage drop over the resistor/capacitor member is equal to that of the inverter (Federal Republic of Germany OS No. 27 40 289). The principle of measurement is in its turn based on the measurement of that change in the resistance of the resistance probe, which is surrounded to a greater or lesser extent by the fluid to be measured, which is brought about at the end of aheating time by a constant current. Ordinarily, in this case the difference between the voltage U.sub.1 at the end of the heating time and an initial voltage U.sub.0 at the beginning of the heating time is detached. In order for the result of this measurement to be independent of the ambient temperature, this prior art specifically provides for the following: At the time of the connection of the resistance probe, the inverter supplies a voltage V-U.sub.0 which charges the capacitor of the resistor/capacitor member. The latter then discharges in accordance with a known law which is determined by the resistance and the capacitance. If the resistance probe heats up during the further course of the heating time, the voltage U increases and the inverter supplies a voltage of the form V-U, which is dependent on the depth of immersion of the resistance probe. By means of the comparator, the time is determined at which the voltage curve of the capacitor (charge curve) which can be represented as a straight line, intersects the curve of the inverted voltage V-U. At a time determined by the program control, the comparator is switched in such a manner that a comparison is effected between the value of the inverted voltage V-U.sub.0 at the terminals of the resistor/capacitor member and the value of the voltage present on the inverter. A monitoring device, which is connected to the output of the comparator, can in this case give off an alarm if a predeterminable threshold value is exceeded. In general, the level of the liquid can be indicated by digital or analog display instruments in accordance with this prior art.
In order to improve the precision of the electrothermal measurement of level compensated for ambient temperature at little expense it has also already been proposed, in a circuit having a comparator which compares a first variable formed from an initial voltage on the resistance probe at the start of a heating time with a second variable formed from a heating voltage at the end of a heating time, the forming of at least one of the two variables being effected in accordance with a time function determined by a resistor and capacitor, to connect the resistor and the capacitor together with an operational amplifier as integrator whose input can be coupled via a sample and hold member for the initial voltage to the resistance probe and whose output is connected to a first input of a comparator. The second input of the comparator is, in this case, coupled with the resistance probe via another sample and hold member directly for the forwarding of the heating voltage at the end of a heating time. A pre-settable backward counter for determining the time between actuation of the second sample and hold member and the switching of the comparator is connected to one output of the comparator. This circuit therefore measures the level as approximately proportional time (German patent application No. P 34 08 824.5 filed Mar. 10, 1984 corresponding to U.S. patent application Ser. No. 710,168 filed Mar. 11, 1985, neither of these documents being prior art).
In order to compensate for the temperature upon the electrothermal measurement of level, it is furthermore known to use a separate compensation resistor which is connected, in series with the resistance probe, to one input of an inverting amplifier. (VDO Paper, Haub et al. SAE Conference on Sensors and Activaters). The compensation resistor is preferably arranged in the vicinity of the resistance sensor and traversed by as small a current as possible in order to avoid heating. If however, the compensation resistor is arranged in the vicinity of the resistance sensor within the container, it is then necessary to lead out an additional connection. Furthermore, it is difficult to find a compensation resistor which has the same temperature coefficient as the measurement sensor.
However, all of the methods and circuits discussed above, have the disadvantage that, in them, noise pulses of the current supply, such as frequently occur in particular in automotive vehicles, have a disadvantageous effect on the result of the measurement.
It is an object of the invention to develop a method of the above-mentioned type in such a way that noise pulses of the supply voltage, which can act differently on the probe voltage at the start of the heating time and at the end of the heating time, distort the result of the measurement as little as possible. The precision and reproducibility of each individual measurement is to be increased by this method.